A need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. A need also exists for a replacement for the conventional cathode-ray tube used in many display devices including computer monitors, conventional and high-definition television receivers and large-screen displays. Both of these needs can be satisfied by display devices that incorporate a light valve that uses as its light control element a spatial light modulator based on a surface-stabilized ferroelectric liquid crystal (SSFLC) material.
A SSFLC-based spatial light modulator is composed of a layer of a SSFLC material sandwiched between two transparent electrodes. One of the electrodes is segmented into an array of pixel electrodes to define the picture elements (pixels) of the spatial light modulator. The direction of an electric field applied between each pixel electrode and the other electrode determines whether or not the corresponding pixel of the spatial light modulator rotates the direction of polarization of light falling on the pixel. The spatial light modulator is constructed as a half-wave plate and rotates the direction of polarization through 90.degree. so that the polarized light transmitted by the pixels of the spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel. The resulting optical characteristics of each pixel of the spatial light modulator are binary: the pixel either transmits light (its 1 state) or absorbs light (its 0 state), and therefore appears light or dark, depending on the direction of the electric field.
To produce the grey scale required for conventional display devices, the apparent brightness of each pixel is varied by temporally modulating the light transmitted by each pixel. The light is modulated by defining a basic time period that will be called the illumination period of the spatial light modulator. The pixel electrode is driven by a drive signal that switches the pixel from its 1 state to its 0 state. The duration of the 1 state relative to the duration of the illumination period determines the apparent brightness of the pixel.
Ferroelectric liquid crystal-based spatial light modulators suffer the disadvantage that, after each time the drive signal has been applied to a pixel electrode to cause the pixel to modulate the light passing through it, the DC balance of the pixel must be restored. This is done by defining a second basic time period called the balance period, equal in duration to the illumination period, and driving the pixel electrode with a complementary drive signal having 1 state and 0 state durations that are complementary to the 1 state and 0 state durations of the drive signal during the illumination period. The illumination period and the balance period collectively constitute a display period. To prevent the complementary drive signal from causing the display device to display a substantially uniform, grey image, the light source illuminating the light valve is modulated so that the light valve is only illuminated during the illumination period, and is not illuminated during the balance period. However, modulating the light source as just described reduces the light throughput of the light valve to about half of that which could be achieved if DC balance restoration were unnecessary. This means that a light source of approximately twice the intensity, with a corresponding increase in cost, is necessary to achieve a given display brightness. Additionally or alternatively, projection optics with a greater aperture, also with a corresponding increase in cost, are necessary to achieve a given brightness.
Recently, a need for reflective light valves based on reflective spatial light modulators has arisen. Reflective spatial light modulators use reflective pixel electrodes and have the advantage that they do not require a transparent substrate. Accordingly, such spatial light modulators can be built on a silicon substrate that also accommodates the drive circuits that derive the drive signals for the pixel electrodes from the input video signal. A reflective light valve has the advantage that its pixel electrode drive circuits do not partially occlude the light modulated by the pixel. This enables a reflective light valve to have a greater light throughput than a similar-sized transmissive light valve and allows larger and more sophisticated drive circuits to be incorporated.
FIG. 1 A shows part of a display device incorporating a conventional reflective light valve 10 that includes the reflective spatial light modulator 25. Other principal components of the light valve are the polarizer 17, the beam splitter 19 and the analyzer 21. The light valve is illuminated with light from the light source 15. The light output by the light valve passes to the output optics 23 that focus the light to form an image (not shown). The light valve, light source and output optics may be incorporated into various types of display device, including miniature, wearable devices, cathode-ray tube replacements, and projection displays.
Light generated by the light source 15 passes through the polarizer 17. The polarizer polarizes the light output from the light source. The beam splitter 19 reflects a fraction of the polarized light output from the polarizer towards the spatial light modulator 25. The spatial light modulator is divided into a two-dimensional array of picture elements (pixels) that define the spatial resolution of the light valve. The beam splitter transmits a fraction of the light reflected by the spatial light modulator to the analyzer 21.
The direction of an electric field in each pixel of the spatial light modulator 25 determines whether or not the direction of polarization of the light reflected by the pixel is rotated by 90.degree. relative to the direction of polarization of the incident light. The light reflected by each pixel of the spatial light modulator passes through the beam splitter 19 and the analyzer 21 and is output from the light valve depending on whether or not its direction of polarization was rotated by the spatial light modulator. The light output from the light valve 10 passes to the output optics 23.
The light source 15 may be composed of LEDs. The LEDs are of three different colors in a color display. Other light-emitting devices whose output can be rapidly modulated may alternatively be used as the light source 15. As a further alternative, a white light source and a light modulator (not shown) may be used. The light modulator modulates the amplitude of the light generated by the light source to define the illumination period and balance period of the spatial light modulator. In a light valve for use in a color display device, the light modulator additionally modulates the color of the light output from the light source.
The polarizer 17 polarizes the light generated by the light source 15. The polarization is preferably linear polarization. The beam splitter 19 reflects the polarized light output from the polarizer towards the spatial light modulator 25, and transmits to the analyzer 21 the polarized light reflected by the spatial light modulator. The direction of maximum transmission of the analyzer is orthogonal to that of the polarizer in this example.
The spatial light modulator 25 is composed of the transparent electrode 33 deposited on the surface of the transparent cover 37, the reflective electrode 35 located on the surface of the semiconductor substrate 39, and the surface-stabilized ferroelectric liquid crystal (SSFLC) layer 31 sandwiched between the transparent electrode and the reflective electrode. The reflective electrode is divided into a two-dimensional array of pixel electrodes that define the pixels of the spatial light modulator and of the light valve. A substantially reduced number of pixel electrodes is shown to simplify the drawing. For example, in a light valve for use in a large-screen computer monitor, the reflective electrode could be divided into a two-dimensional array of 1600.times.1200 pixel electrodes. An exemplary pixel electrode is shown at 41. Each pixel electrode reflects the portion of the incident polarized light that falls on it towards the beam splitter 19.
A drive circuit (not shown), which may be located in the semiconductor substrate 39, applies a drive signal to the pixel electrode of each pixel of the spatial light modulator 25. The drive signal has two different voltage levels, and the transparent electrode 33 is maintained at a fixed potential mid-way between the voltage levels of the drive signal. The potential difference between the pixel electrode and the transparent electrode establishes an electric field across the part of the liquid crystal layer 31 between the pixel and transparent electrodes. The direction of the electric field determines whether the liquid crystal layer rotates the direction of polarization of the light reflected by the pixel electrode, or leaves the direction of polarization unchanged.
The reflective spatial light modulator 25 is structured as a quarter-wave plate in contrast to a transmissive spatial light modulator, which is structured as a half-wave plate. This difference arises because light passes through the reflective spatial light modulator twice, once before and once after reflection by the reflective pixel electrodes. The thickness of the layer of ferroelectric liquid crystal material in the liquid crystal layer 31 is chosen to provide an optical phase shift of 90.degree. between light polarized parallel to the director of the liquid crystal material and light polarized perpendicular to the director. The liquid crystal material is preferably a Smectic C* ferroelectric liquid crystal material having an angle of 22.5.degree. between its director and the normal to its smectic layers. Reversing the direction of the electric field applied to such a liquid crystal material switches the director of the material through an angle of about 45.degree.. Consequently, if the director is aligned parallel to the direction of maximum transmission of the analyzer 21 with one polarity of the electric field, reversing the direction of the electric field will rotate the direction of polarization of light reflected by the pixel through 90.degree.. This will align the direction of polarization of the light perpendicular to the direction of maximum transmission of the analyzer, and will change the pixel from its 1 state, in which the pixel appears bright, to its 0 state, in which the pixel appears dark.
In a miniature, wearable display, the output optics 23 are composed of an eyepiece that receives the light reflected by the reflective electrode 35 and forms a virtual image at a predetermined distance in front of the user (not shown). In a cathode-ray tube replacement or in a projection display, the output optics are composed of projection optics that focus an image of the reflective electrode on a transmissive or reflective screen (not shown). Optical arrangements suitable for use as an eyepiece or projection optics are well known in the art and will not be described here.
Since the direction of maximum transmission of the analyzer 21 is orthogonal to the direction of polarization defined by the polarizer 17, light whose direction of polarization has been rotated through 90.degree. by a pixel of the spatial light modulator 25 will pass through the analyzer and be output from the light valve 10 whereas light whose direction of polarization has not been rotated will not pass through the analyzer. The analyzer only transmits to the output optics 23 light whose direction of polarization has been rotated by pixels of the spatial light modulator. The pixels of the spatial light modulator will appear bright or dark depending on the direction of the electric field applied to each pixel. When a pixel appears bright, it will be said to be in its 1 state, and when the pixel appears dark, it will be said to be in its 0 state.
The direction of maximum transmission of the analyzer 21 can alternatively be arranged parallel to that of the polarizer 17, and a non-polarizing beam splitter can be used as the beam splitter 19. In this case, the spatial light modulator 25 operates in the opposite sense to that just described.
To produce the grey scale required by a display device notwithstanding the binary optical characteristics of the pixels of the light valve 10, the apparent brightness of each pixel is varied by temporally modulating the light reflected by the pixel, as described above. The drive circuit (not shown) for each pixel of the spatial light modulator determines the duration of the 1 state of the pixel in response to a portion of the input video signal 43 corresponding to the location of the pixel in the spatial light modulator.
FIGS. 1B-1F illustrate the operation of the exemplary pixel 41 of the conventional light valve 10 shown in FIG. 1A during three consecutive display periods. The remaining pixels operate similarly. In one embodiment of a conventional light valve, each display period corresponded to one frame of the input video signal 43. In another embodiment, each display period corresponded to a fraction of one frame of the input video signal. Each display period is composed of an illumination period (ILLUM) and a balance period (BALANCE) having equal durations, as shown in FIG. 1B.
FIG. 1C shows the drive signal applied to the exemplary pixel electrode 41. The transparent electrode 33 is held at a voltage level of V/2, so that changing the voltage level on the pixel electrode from 0 to V reverses the direction of the electric field applied to the ferroelectric liquid crystal layer 31. The level of the drive signal is V for a first temporal portion 1 TP of each illumination period. The level of the drive signal is 0 for the second temporal portion 2TP constituting the remainder of the illumination period, and also for the first temporal portion 1TP of the subsequent balance period. The first temporal portion of the balance period has a duration equal to the first temporal portion of the illumination period. However, the level of the drive signal is 0 during the first temporal portion of the balance period, whereas the level of the drive signal is V during the first temporal portion of the illumination period. Finally, the level of the drive signal changes to V for the second temporal portion 2TP constituting the remainder of the balance period. Consequently, during the balance period, the level of the drive signal is 0 and V for times equal to the times that it was at V and 0, respectively, during the illumination period. As a result, the electric field applied to the liquid crystal material of the pixel averages to zero over the display period.
In the example shown, the duration of the first temporal portion 1TP of the drive signal is different in each of the three illumination periods. The duration of the first temporal portion, and, hence, of the second temporal portion, of each illumination period depends on the voltage level of the corresponding sample of the input video signal 43.
FIG. 1D shows the effect of the spatial light modulator 25 on the direction of polarization of the light impinging on the analyzer 21. The direction of polarization is indicated by the absolute value of the angle a between direction of polarization of the light impinging on the analyzer and the direction of maximum transmissivity of the analyzer. The analyzer transmits light having an angle .alpha. close to zero and absorbs light having an angle .alpha. close to 90.degree.. In each display period, the angle a has values corresponding to the pixel being bright and dark for equal times due to the need to restore the DC balance of the pixel.
FIG. 1E shows the modulation of the light source 15. The light source is ON throughout the illumination period of each display period, and is OFF during the following balance period.
FIG. 1F shows the light output from the exemplary pixel of the light valve 10 controlled by the pixel electrode 41. Light is output from the pixel only during the first temporal portion of the illumination period of each display period. No light is output during the second temporal portion of the illumination period. Moreover, no light is output during the balance period of the display period because the light source 15 is OFF during the balance period. The light source being OFF for half the display period substantially reduces the perceived maximum brightness of the pixel, and of any image generated by a display device incorporating such a conventional light valve.
The need to maintain the DC balance of the ferroelectric liquid crystal material of the spatial light modulator 25 means that the reflective light valve just described suffers from a similar light throughput problem to the transmissive light valves described above. Consequently, what is needed is a reflective ferroelectric liquid crystal-based light valve that can remain illuminated during the balance period so that the light throughput of the light valve can be approximately twice that of a conventional light valve.